If the President of the United States meets with the Prime Minister of Great Britain to finalize negotiations of a treaty allowing the United States to use British territory to establish a military base, he is performing his role as Commander in Chief.
Among the different power assigned to the President of the United State, he also acts as commander in chief of the armed force. Being Commander in Chief, he can take important military decisions. He can declare war, conclude treaties and make any military-related policies.
#SPJ4